1. Field of the Invention
The invention relates to means for monitoring seagoing characteristics of a vessel or ship, and more particularly it relates to systems for automated monitoring of the trim and stability of a vessel.
The invention can be utilizd on ships of all kinds having displacement of at least one thousand register tons, such as tankers, timber carriers, container carriers, ships of the ro-ro and ro-flow types, ore and bulk cargo carriers, on fishing trawlers and offshore platforms.
It is generally known that in order to ensure safe sailing, the crew of a ship has to be aware of the performance characteristics of the ship at any given moment, e.g. the ship's seagoing and strength characteristics, as well as of the current situation in the complex vessel-cargo-environment dynamic system, particularly, in emergencies that may eventually arise in everyday service.
2. Description of the Prior Art
There are known various systems for monitoring the trim, stability and general strength of a vessel.
Thus, there are systems for monitoring the general strength of a vessel based on evaluation of the general bending moment and of the shearing forces or strain produced by general bending of the vessel's hull.
One of the known systems (installed on M/V "Olympic Challenge"; 65,000 ton dwt) comprises a projector mounted amidships and directing a light pencil onto a mirror accommodated on the aft super-structure. The reflected beam is sent by the mirror onto a photocell arranged adjacent to the projector. Should the reflected beam deflect, the photocell initiates a signal sent to a mechanism for rotating the mirror, so as to return the beam to its home position. The angle of rotation of the mirror serves as the output signal of the ship sag sensor. However, as bending moments and shearing forces are different at various sections of the ship's hull, the abovedescribed technique would not yield adequately accurate results.
There is further known a strength monitoring system acting on the same principle as the one described above. This system is operable for determining the bending moments and shearing forces from the values of the draft of the ship measured at several sections. This system is even less accurate on account of imperfection of available draft sensors, as their absolute error in measuring the draft more often than not is as great as the maximum sag of the ship. Moreover, the two abovementioned systems do not monitor the stability of a ship.
Furthermore, there are known systems for monitoring the general strength of a ship based on direct measurements of strain in the ship's hull with the use of strain gauge sensors (cf. "Wedar" system developed in Norway). The system is intended for measuring the wave impact load at sea. The external load is varied by altering the ship's course and speed. The system includes the starboard and port deck-mounted sensors measuring the total strain induced by the bending moment amidships, a bow accelerometer measuring vertical motion of the ship, an electronic amplification device and a unit for processing the values being measured situated in the wheelhouse, and various alarms. When the strain level reaches a threshold value, a warning signal is generated. The system fails completely to monitor the trim and stability characteristics; moreover, various design deficiencies of the system's hardware have resulted in its limited use.
There is still further known a system for monitoring the trim and stability of a vessel (E. V. Naidenov, "Kontrol posadki i ostoichivosti sudna", 1983, "Transport" (Moscow), pp. 111-112). To determine the draft values, two draft gauges of the hydrostatic type are mounted at the ship's bow and stern. To determine the metacentric height from a heeling test, it is necessary to know the inclining moment, the displacement of the ship and the heel angle increment which the ship attains due to the inclining moment applied thereto.
The "Intering" system provides solely for automatic heeling of the ship, i.e. for driving the ballast fluid into the heeling tank by a compressor.
The heeling moment value always remains constant (about 50-60 tm) depending on the type of the ship. The value of the increment of the heel angle is obtained from an inclination meter which is a common-type level gauge with a two-metre base. The accuracy of determination of an increment to the heel angle attainable with this instrument is about 0.1 arcuate degree. This known system is characterized by relatively low accuracy and the complicated procedure of determining the metacentric height. The low accuracy is caused by the system incorporating the inclination meter with 0.1 arcuate degree divisions and by the constancy of the heeling moment which, in case of a small metacentric height, can result in the ship's heel attaining the highly undesirable value up to 8.degree. that may cause displacement of the ship's cargo, because in most cases the heeling procedure is carried out after cargo-handling operations, prior to the ship's departure or while securing the cargo; furthermore, heel angles of this magnitude reduce the accuracy of metacentric height computation. Thus, with a heel angle of 8.degree. and a small metacentric height the relative error can be as high as 10 percent. With a great value of the metacentric height, on the other hand, the constant heeling moment causes a heel angle whose value is too low --as low as 0.3.degree.-0.4.degree.--and is commensurate with an error in measuring the heel angle increment. This causes considerable errors in computation of the metacentric height, as great as 25 percent. Therefore, employment of systems of the type being discussed, firstly, affects the economy of ship's operation when a computation error results in the metacentric height being unjustly evaluated too high, and, secondly, may even cause an emergency situation when the computation error drifts towards the minimum value of the metacentric height.
There is further known a system for automated monitoring of the trim and stability of a vessel (E. V. Naidenov, "Kontrol'posadki i ostoichivosti sudna", 1983, "Transport" (Moscow), pp. 115-121). The system comprises a sensor monitoring the value of the draft of the vessel, a heel angle sensor, a heeling subsystem with manually and automatically operated control valves and its power supply unit. The sensors are connected to a signal conditioner which is interconnected with a computer determining the value of the metacentric height. The operation of this system is characterized by the following. If the ship's stability is low, the operation of the system could cause a considerable list of the vessel which is intolerable in practice, whereas when the stability is high, the system fails to produce the necessary heeling of the ship, which increases the error in determining the metacentric height. Moreover, the system's response is not adequately quick.
When the heeling operations are conducted at sea, the accuracy of determining the displacement value is low on account of the absence of compensation for dynamic head of water. Neither does the system account for the influence of such factors as the trim difference and submerged depth. Besides, the system is devoid of facilities for monitoring the general strength of a vessel.